Certain types of systems for controlling the air temperature within an enclosure employ a reversible AC motor which is under the control of a thermostat. The motor may position a damper which regulates flow of heating or cooling air into the enclosure or may position a valve which regulates the flow of heating or cooling water to a heat exchanger within the enclosure. Such motor control thermostats typically include two temperature responsive switches, one of which controls motor shaft rotation for opening the valve or damper and another controlling the closing of same. In the conventional embodiments, the switches are carried on the end of a bimetal strip which tilts in response to temperature changes, thereby changing the angular orientation of the switches. These switches open and close depending on their angular orientation, and in this way respond to temperature changes.
It is preferable in controlling the position of the motor shaft, to minimize the amount of movement of the shaft and damper or valve. This reduces operating time of the motor and damper or valve, reducing wear and lengthening the operating life of the devices.
To reduce operating time of the motor and to prevent overshoot of the temperature within the controlled enclosure, it is customary to use so-called anticipator resistors to apply heat to the bimetal strip. When the temperature falls below the lower control limit and the shaft is rotated so as to increase enclosure temperature, a measure of additional heat called anticipation heat is applied to the bimetal strip to cause the strip's ambient temperature to rise more rapidly than the enclosure temperature. Thus the increase in the influx of warmer air or the decrease in the influx of cooler air is reduced by the bimetal strip's early response to the anticipation heat. That is, the thermostat anticipates the warming of the enclosure by the anticipation heat applied to it and causes the switch whose closing causes the enclosure to warm, to open before the enclosure temperature rises above the upper temperature control limit. Similarly, when the enclosure temperature rises above the upper control limit, and cooling is applied to the enclosure, it is necessary to cool the bimetal strip more rapidly than the enclosure itself to prevent cooling the enclosure to below the lower control limit. Since it is inconvenient to actually cool the bimetal strip, instead the thermostat is designed to provide the bimetal strip when both the heating and cooling switches are open, with a level of anticipation heat which is lower than that applied when the heating switch is closed. When the enclosure temperature rises above the upper control limit and the switch controlling cooling closes, then this lower level of anticipation heat is removed from the bimetal strip, in effect applying "cooling" to it.
To apply anticipation heat as described with the anticipator resistors, it is the practice to draw the (lower level of) anticipation current flowing when both control switches are open, through the motor. It has been found for some of these motor designs that this relatively small current (typically 5-15 ma.) is nonetheless capable of causing the motor shaft position to slowly drift in response to any unbalanced torque on the shaft. This results in the need to reposition the motor shaft much more frequently than if such current were not constantly flowing through the winding. This current has also been found to reduce motor torque when driving the motor during the time that the heating switch is closed and the higher level of anticipation heat is required.
It is easiest to explain in greater detail these characteristics of such a conventional thermostatically controlled system with reference to FIG. 1. The motor 10 is shown as having a shaft 15 which is driven, typically through a gear drive not shown, in the clockwise (forward) direction as viewed in the drawing by alternating current applied to a field winding 11 and in the counterclockwise (reverse) direction by current applied to a field winding 12 as the legends CW and CCW indicate. Shaft 15 is mechanically connected (the mechanical connection is symbolized by the dotted line) to a damper 16 within an air duct 17 so that forward rotation of shaft 15 causes the damper to rotate clockwise to a more open setting which is maximum when the damper 16 is parallel to the duct centerline, with reverse rotation of shaft 15 achieving the opposite effect. Cooling or heating air flow into the enclosure 18 supplied by a fan not shown is thus controlled by damper 16. Capacitor 13 is a conventional phase capacitor which couples the two windings 11 and 1 to each other to form the rotating magnetic field necessary to induce rotation of shaft 15. Power for motor 10 is supplied at terminals 36 and 37 and typically will be 24 volts AC supplied by the secondary of a transformer not shown.
An enclosure 18 is shown diagrammatically as having within it circuitry comprising a conventional thermostat 9 whose purpose is to control the position of damper 16 so as to regulate the temperature within enclosure 18 according to the occupants' wishes. Of course the reader understands that the typical enclosure 18 will be a room with a thermostat 9 which occupies a very small part of one of the walls defining the room. The implementation shown is for cooling a room with cooling air flowing in duct 17, but by merely reversing the thermostat connections to windings 11 and 12, the thermostat will operate in heating mode assuming heated air is available in the duct.
Power is switched to motor 10 by the thermostat's mercury level switches 19 and 29 carried on the arm 26 forming the outer end of a temperature sensitive element, typically a bimetal strip 24. As is well known, such bimetal strips tend to change the total angular amount they are coiled as the ambient temperature changes, and it will be assumed that the bimetal strip 24 tends to unwind (straighten) with decreasing temperature and wind more tightly with increasing temperature as viewed in FIG. 1. The effect of this is to cause the arm 26 of strip 24 to rotate clockwise with increasing temperature and rotate counterclockwise with decreasing temperature. The entire bimetal strip 24 along with switches 19 and 29, is carried on a shaft 28 which may be rotated by the room's occupant by a mechanism not shown to change the angular position of arm 26 for a particular temperature. It is usual to carry switches 19 and 29 on a frame, omitted for simplicity's sake here, which is attached to arm 26. Such a frame may be adjustable to change the amount of displacement of arm 26 between the angles at which switches 19 and 29 close. This difference in the closing angles for the switches sets the width of the control range in which no repositioning of the damper 16 is necessary. Electrical connections to these switches 19 and 29 are through flexible leads symbolized by the pigtails 27 which isolate arm 26 from any external torque which might affect its motion in response to temperature changes.
Each of the mercury switches 19 and 29 includes a glass or plastic capsule 20 or 30 having a small mercury ball or globule 21 or 31 within it and pairs of internal contacts 22 and 23 or 32 and 33. Contacts 22 and 23 control current flow to winding 11 and contacts 32 and 33 control current flow to winding 12. In the attitude shown in FIG. 1, neither switch 19 nor 29 is conducting. Cooling bimetal strip 24 to a first selectable temperature causes bimetal strip 24 to unwind slightly and mercury ball 31 to roll to the left to complete the connection between contacts 32 and 33 and close switch 29 If bimetal strip 24 then warms slightly, mercury ball 31 returns to its position at the right end of capsule 30 opening switch 29. If bimetal strip 24 warms to a second selectable temperature, mercury ball 21 rolls to its left and forms an electrical connection between contacts 22 and 23 thereby closing switch 19. Of course, subsequent cooling of bimetal strip 24 breaks the connection between contacts 22 and 23 and opens switch 19. The relative angular position of each of these capsules 20 and 30 with respect to each other as mounted on arm 26 and the frame is such as to have a temperature range of a few, perhaps 2, degrees (Fahrenheit) within which neither of the pairs of contacts 22, 23 and 32, 33 are connected to each other by their respective mercury balls 21 or 31. Contacts 22 and 23 are connected directly to forward winding 11 to form a first series motor circuit to control the flow of electrical power for driving motor 10 forward. Contacts 32 and 33 are connected directly to reverse winding 12 to form a second series motor circuit to control the flow of electrical power for driving motor 10 in reverse. The first and second selectable temperatures are dependent on the angular position of shaft 28 which is under the control of the enclosure's occupants.
For the implementation in FIG. 1 operating in the air conditioning mode assumed, increasing air flow in duct 17 cools enclosure 18. If temperature should rise within enclosure 18 to the second selectable temperature then, additional air flow into the enclosure is necessary to cool the enclosure. This requires motor shaft 15 to be driven in a forward or clockwise direction to open damper 16 an additional amount. The increasing temperature within enclosure 18 and adjacent bimetal strip 24 closes the circuit between contacts 22 and 23 which allows power to flow to winding 11 from terminals 36 and 37. Motor shaft 15 rotates clockwise to admit more cool air through duct 17 and the temperature within enclosure 18 and adjacent bimetal strip 24 begins to drop. When the enclosure temperature rises above the first selectable temperature, arm 26 then rotates counterclockwise slightly, the connection between contacts 22 and 23 is broken, and no further change in shaft 15 positioned is called for. No further change is then theoretically necessary in the position of damper 17 while the thermal load on enclosure 18 is unchanged.
Similarly, if temperature falls to the first selectable temperature within enclosure 18, bimetal strip 24 reacts to this condition by unwinding sufficiently to cause mercury ball 31 to connect contacts 32 and 33 causing current to flow through the reverse winding 12 of motor 10 and damper 16 to close. Temperature in enclosure 18 and adjacent bimetal strip 24 begins to rise in response to the reduced flow of cooling air, switch 29 opens, and no further change in the position of damper 16 is necessary for a time.
It has long been known that it is necessary to use anticipator resistors 25 and 35 to prevent overshoot of the temperature within the enclosure 18 from the first and second selectable temperatures defining the ends of the control band and sensed by the bimetal strip 24 and its associated switches 19 and 29. This overshoot occurs principally in these control systems because the damper position reaches the steady state position for maintaining the temperature within the control range much more quickly than the enclosure temperature reaches the control range. If the enclosure temperature was alone relied on to control conduction of the switches 19 and 29, the damper would be driven far beyond the proper steady state control position.
These anticipator resistors 25 and 35 are placed in juxtaposition with the bimetal strip 24 so that heat generated by current flow through them is applied to the bimetal strip 24 and causes it to wind slightly more tightly than it otherwise would if the anticipation heat were not to be provided. Anticipation heat is provided by resistor 35 whenever the low temperature switch 29 is closed to rotate damper 16 to a more closed position and admit less cooling air to enclosure 18. For the high temperature switch 19, anticipation heat is provided to bimetal strip 24 at all times except when switch 19 is closed, shunting resistor 25 while driving damper 16 closed to admit less cooling air to enclosure 18. Note that anticipation heat is applied from both resistors 25 and 35 when switch 29 is closed, since switch 19 is always open when switch 29 is closed. It can thus be seen that three different levels of anticipation heat are provided to bimetal strip 24. When switch 29 is closed, the maximum amount of heat, from both resistors 25 and 35, is provided. When neither switch 19 nor 29 is closed, a lesser amount of heat from resistor 25 only is provided, and when switch 19 is closed, then no anticipation heat is provided.
The anticipation heat level provided when resistor 25 only is conducting tends to cause both switches 19 and 29 to close, for a given shaft setting, at a slightly lower temperature of the air within enclosure 18 than if this anticipation heat was not present. This poses no problem in practice, because the scale on the shaft 28 adjustment mechanism is factory calibrated to compensate for this effect. When temperature falls within enclosure 18 and switch 29 closes (driving damper 16 closed), the additional anticipator heat provided by resistor 35 causes additional heating of bimetal strip 24 and switch 29 to open at a lower temperature of enclosure 18. The value of resistor 35 is selected to cause switch 29 to open before the damper 16 has moved past its optimal position where the enclosure 18 temperature will be held within the control range without further damper movement. The removal of anticipation heat from resistor 25 when switch 19 is closed simulates cooling of bimetal strip 24 and in essence anticipates the cooling of enclosure 18 which arises from closing of switch 19. The value of resistor 25 must be selected so that its removal causes switch 19 to open before damper has moved past the optimal position described above. In both the heating and the cooling cases, the overall temperature within enclosure 18 is held by the presence or absence of the anticipation heat, accurately between the two selectable temperatures defining the control range.
As can be seen from the circuit of FIG. 1, current flow in resistor 25 flows also through winding 11 when switch 19 is open. For some types of loads such as damper 16, the torque loading on motor 10 with motor 10 unpowered may be unbalanced, for example because of gravity. In such cases for some types of motors 10, the anticipation current through resistor 25 and winding 11, even though small, is enough to cause shaft 15 and its load to slowly change position, changing the flow of cooling (or heating) air and causing the enclosure temperature to change. This means that actual closing of a switch to correct the temperature is necessary earlier than would otherwise be required. To reduce wear, a design will ideally cause motor shaft 15 to be rotated only in response to changes in heating or cooling loads on enclosure 18 which cause one or the other of switches 19 and 29 to close, rather than on changes in the shaft 15 position resulting from drift in its position.
Another disadvantage with the design of FIG. 1 is that for some applications there is no value for anticipator resistor 35 which adequately heats bimetal strip 24 to provide sufficient anticipation heat for accurate control when switch 29 is closed. This is because there is not enough voltage available from the 24 VAC control transformer to provide the series connection of resistor 35 and winding 12 with adequate heating of bimetal strip 24 and sufficient current flow through winding 12 for rated torque.
A further problem with this design is that the amount of anticipation heat generated by each of the resistors 25 and 35 is dependent on the number of motors 10 which are controlled by the thermostat 9 and also on the impedance of these motors. It is sometimes desirable to be able to control several motors with a single thermostat as this reduces the number of thermostats required. It is also desirable that the anticipation heat be independent of the characteristics of the particular motor or motors controlled by the thermostat. Because all of the current for anticipator resistors 25 and 35 flows through the windings of motor 10, obviously changing the number or type of motors controlled by the thermostat will change the amount of current flowing through the anticipator resistors 25 and 35 when switch 29 is closed, possibly necessitating a field alterable design for thermostat 9. This is because the amount of anticipation heat required by the thermostat cannot be arbitrarily changed without adversely affecting operation of the system. The additional complexity added by dependence of the anticipation heat on the motor load results in additional cost and creates the possibility that inexpert installation will result in degraded operation of the system.
All of these problems make a new approach to the design of this thermostat desirable.